The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at rapid deceleration of the vehicle.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of the engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting the vehicle, the transmission ratio is set at a maximum value. When an accelerator pedal is depressed and engine speed becomes higher than a set value, the clutch is engaged to start the vehicle. When the vehicle speed and engine speed increase along a maximum transmission ratio line l.sub.L and reach set values under a driving condition, the transmission ratio starts to change (to upshift) at a point A of FIG. 5. At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed along a line l.sub.U and finally reaches a minimum transmission ratio line l.sub.H. When the accelerator pedal is released, engine speed and vehicle speed reduce along the line l.sub.H. When both speeds reach a point B, the transmission ratio begins to increase. Thus, the transmission ratio is increased (downshifted) along a line l.sub.D and reaches the maximum transmission ratio line l.sub.L.
In such a transmission, when the vehicle is rapidly decelerated, the vehicle is stopped before the transmission ratio reaches the maximum ratio line l.sub.L. A dotted line l.sub.1 in FIG. 5. shows the variation of the transmission ratio at a rapid deceleration. When the belt stops when the vehicle is stopped, the movable conical discs can not be shifted in the downshift direction, because of large friction between the belt and discs. As a result, the belt stays in a lower ratio position lower than the maximum transmission ratio position. In such a state, when the clutch is engaged in order to re-start the vehicle, the pulleys being to rotate. At that time, the movable conical discs are quickly shifted to move the belt to the maximum ratio position. Due to the movement of the belt, starting of the vehicle is delayed and the quick shifting of the discs causes wearing of the belt.